Radiation measurement systems (Radiation Spectrometers) are used to obtain radiation spectra or counting information of radiation (charge particles, photons, neutrons) that interact with a radiation detector. The signal from the radiation detector is a short current pulse that delivers a very small charge. This charge is converted to a voltage pulse using charge-sensitive preamplifier. Preamplifiers that are based on resistive-feedback produce pulses with short rise time and slow exponential decay—an exponential tail pulse. The exponential tail pulse passes through a pulse shaping network that produces short pulses with good signal-to-noise ratio. The first stage of the pulse shaping network performs differentiation in order to produce a pulse with shorter decay time than the pulse from the preamplifier. FIG. 1 shows a typical arrangement to differentiate the signal from the preamplifier.
The radiation detector (10) produces short current pulse I(s) (12) that is sensed by the preamplifier (20). The preamplifier has an amplifier (18) and feedback network of resistor (16) with value Rp and capacitor (14) with value Cp. The output of the preamplifier (20) is an exponential tail pulse vp(s) (22). The exponential tail pulse (22) pass through an CR differentiator (30) with capacitor (24) and resistor (26) with values Cd and Rd respectively. The combined response of the preamplifier (20) and the CR differentiator (30) then will result in a differentiated pulse vd(s) (28) with an undesirable undershoot. The differentiated pulse can be represented as an exponential tail pulse with decay time constant equal to the Cd*Rd minus a fraction of the preamplifier exponential tail pulse with decay time constant Cp*Rp. In other words, despite of the differentiation the long tail is still present in the differentiated pulse (28).
The undershoot of the differentiated pulse (28) can cause a significant spectral distortion, especially at high counting rates. Therefore, the goal of Pole-Zero Compensation is to remove the undershoot from the differentiated pulse. This goal is easily achieved by adding a fraction of the exponential tail pulse (22) to the differentiated pulse (28). FIG. 2 shows a classic Pole-Zero Compensation circuit using manual adjustment. The attenuator (32) is fed with the same signal as the CR-differentiator—the exponential tail pulse (22). The attenuator (32) is manually adjusted (trim pot, variable resistor) to provide an attenuated tail pulse G*vp(s) (34). G is the attenuation coefficient and it is bounded between zero and one −0<G<1. The differentiated pulse (28) and the attenuated tail pulse (34) are added together by the analog adder (40). The analog addition can be done using passive resistive networks or active, amplifier based, summing circuit. The Pole-Zero exponential pulse v(s) (38) is delivered at the output of the analog adder (40). Depending on the value of G there are three possible categories of shapes for the Pole-Zero exponential pulse (38).
FIG. 3 shows cases of Overcompensated (60), Compensated (62), and Undercompensated (64) Pole-Zero exponential pulses. At the proper setting of G, the attenuated tail pulse (34) will completely compensate for the undershoot of the differentiated pulse (28), resulting in single time-constant (Cd*Rd) Pole-Zero exponential pulse (Compensated Pole-Zero Pulse). When G is greater than the optimum compensating value, the Pole-Zero exponential pulse will exhibit an overshoot (Overcompensated Pole-Zero Pulse). When G is less than optimum compensating value, the Pole-Zero exponential pulse will exhibit an undershoot (Undercompensated Pole-Zero Pulse).
When the Pole-Zero exponential pulses pass through linear pulse shaper (analog or digital), the overshoot/undershoot features are also present in the resulting shaped pulses. FIG. 4 illustrates shaped pulses resulting from Pole-Zero exponential pulses. An Overcompensated Pole-Zero pulse (60) will cause an Overcompensated Shaped Pulse (70) that will exhibit a longer decaying tail. A Compensated Pole-Zero pulse (62) will cause the fastest recovery to the baseline of the Compensated Shaped Pulse (72). The undershoot of the Undercompensated Pole-Zero pulse (64) will propagate to the Undercompensated Shaped Pulse (74).
The automatic Pole-Zero compensation uses digitally controlled attenuator (52) as shown in FIG. 5. The attenuation coefficient G is proportional to a digital value D (50). By changing the digital value (50), one can control the digitally attenuated tail pulse G*vp(s) (54). The digitally controlled attenuator (52) could be a digitally controlled potentiometer, amplifier with digitally controlled gain or multiplying digital-to-analog converter (MDAC).
The automatic compensation of the Pole-Zero requires means to determine (estimate) whether the Pole-Zero is properly compensated, overcompensated or undercompensated. U.S. Pat. No. 4,866,400 (FIG. 6) uses analog boxcar average (82) connected to the output of pulse shaper (80). A comparator that is a part of the control circuit (84) examines the output of the Boxcar Average (82) and decides in which direction to change the digital value D (50). The condition for overshoot or undershoot is determined by examination of the baseline of pulse shaper (80) after each shaped pulse. U.S. Pat. No. 5,872,363 extends the same technique into the digital domain. In both cases, the correct determination of the Pole-Zero state depends on the DC offset of the baseline. The DC offset of the base line causes errors that limit the accuracy of the automatic pole-zero compensation.
U.S. Pat. No. 6,374,192 (FIG. 7) uses a gated integrator (90), operating in the analog domain, to determine the compensation state of the Pole-Zero network. The conditions for overshoot or undershoot is determined by examining the slope of the flat part of the gated integrator pulse. The slope is determined from two amplitude measurements performed by an ADC (92). A Control Circuit (94) increments or decrements the digital value (50) when the slope is negative or positive respectively. As in the previous two cases the accuracy is affected by the DC offset of the baseline of the pulse shaper (80). A DC baseline offset will cause a non Pole-Zero related slope at the output of the gated integrator (90). To eliminate the effects of the baseline DC offset and the detector charge collection time on the automatic Pole-Zero compensation a new method and apparatus were developed.